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Integr Comp Biol. 2008 Nov;48(5):668-80. doi: 10.1093/icb/icn048. Epub 2008 Jun 2.

The midline, oral ectoderm, and the arch-0 problem.

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  • 1Institute of Neuroscience, University of Oregon, Eugene, OR 97403, USA.


In most versions of theories of the segmentation of the vertebrate head, a premandibular segment is present rostral to the jaw-forming mandibular segment. These theories posit that in ancient fishes this segment included a gill and a gill-supporting skeleton, which then was modified to support the anterior brain. However, we find no recent evidence for existence of such a premandibular segment. Rather, new findings from studies of fate mapping and gene expression show that the "premandibular" territory is in fact the maxillary region of the mandibular arch. A signaling cascade, beginning with dorsal midline mesoderm in the gastrula and relayed through neural ectoderm and then oral ectoderm, greatly expands the skeletal derivatives of maxillary neural crest in a manner fully consistent with the Gans-Northcutt theory of the vertebrate new head.

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